(1) Field of the Invention
The present invention relates to a calibration circuit. More particularly the system generates a plurality of frequencies simultaneously at repeatable levels for the calibration of hydrophones. (2) Description of the Prior Art
Prior to the present invention the time required by the instrumentation to perform the hydrophone calibration in a Deployable Noise Measurement (DNM) Program was lengthy and costly. The Deployable Noise Measurement Program processes torpedo radiated noise of the Navy's research and development torpedoes. The calibration of the hydrophones costs ship time and labor hours during the conduct of an operation.
The past Deployable Noise Measurement Program system used a programmable analog signal generator to separately send twenty-five sequential 7/8-octave band (center frequency) tones to the calibration pad, hereinafter called CAL PAD of hydrophones during the calibration. The tones started at 50 Hz and ended at 12.5 kHz. Each tone was sent for a duration of 30 seconds. The result was a relatively slow calibration procedure for each hydrophone. Each hydrophone has two channels; a direct signal channel and an F.M. channel; to cover the appropriate range and signal to noise ratio. If two hydrophones are used, each with two channels, it takes about one hour to calibrate the two hydrophones at 25 different frequencies in the 50 Hz-12.5 kHz range. This prior art method also requires the tape playback calibration to be synchronized.